1. Field of the Invention
The present invention relates to the manufacture of arrays of vertical cavity surface emitting lasers (VCSELs) emitting light in the infrared range, and more particularly to the manufacture of such VCSEL arrays using vapor phase epitaxial growth techniques such as Metal Organic Chemical Vapor Deposition (MOCVD), Metal Organic Vapor Phase Epitaxy (MOVPE), or the like.
2. Art Background
It is often advantageous to manufacture light-emitting semiconductor devices in the form of arrays. For example, light emitting devices such as light-emitting diodes (LEDs) and vertical cavity surface emitting lasers (VCSELs) can be manufactured in the form of arrays, wherein the devices are epitaxially grown on a single substrate, processed and auto-tested as a whole wafer. Such devices have numerous benefits over light-emitting devices which cannot be auto-tested as whole wafer, such as edge emitting lasers.
For VCSELs, it is critically important that they exhibit device-to-device uniformity in their operating characteristics. More specifically, it is critically important that the VCSELs in an array exhibit uniform light output characteristics at a fixed input current. Where VCSEL arrays are used in data communications settings, light output power for each laser in an array must not be permitted to exceed an eye-safety limit (ESL) but yet have sufficient output power to maintain a bit-error rate below a specified maximum. Thus, predictability of VCSEL operation is crucial. VCSEL operation will vary as a function of many variables, including temperature, age, threshold current, etc. To meet the requisite specifications of multichannel communications applications, therefore, each variable must be constrained within a budget of variability to meet the operating specifications of an application. The typical budget for device-to-device non-uniformity is in the range of one decibel as constant current arc temperature.
It is neither practical nor cost-effective to drive each VCSEL of an array with a different current to achieve uniform light output power. Techniques similar to those used to maintain constant output power from edge emitters have been adapted for application to VCSEL arrays. For edge emitters, a back facet photodetector is integrated with the edge emitter to sense light which is being emitted from the back facet of the edge emitter. This photodetector creates an electrical control signal which is sensed to provide feedback control to the current level driving the edge emitter. While feedback control circuits have been invented to control the fluctuation of output power of arrayed VCSELs due to changes in substrate temperature (see U.S. application Ser. No. 08/302,313), such schemes add significantly to the cost of the VCSEL arrays. Moreover, these techniques do not compensate for variation in the electrical characteristics of each VCSEL over the array. The use of feedback control to eliminate variations in electrical uniformity would require a photodetector and feedback circuit for each individual VCSEL in the array. The added cost for such a scheme would be prohibitive.
Thus, from a cost perspective it is most desirable to drive each VCSEL of an array with the same drive current, and it is desirable that the light output power of each of the individual VCSELs be virtually identical as a function of the drive current. We have discovered that VCSEL arrays, designed to emit radiation in the infrared portion of the spectrum, and manufactured using vapor phase epitaxial growth techniques such as metal organic vapor phase epitaxy (MOVPE) or metal organic chemical vapor deposition (MOCVD) exhibit device-to-device array uniformity which is inferior to the uniformity which is achieved when such VCSEL arrays are manufactured using a molecular-beam epitaxy (MBE) process.
FIG. 1 illustrates the light output power and voltage versus drive current (LIV) characteristics of each of five VCSELs forming part of an eight-VCSEL array manufactured using MBE. FIG. 2 illustrates the LIV characteristics for each of five VCSELs which have been manufactured using an MOCVD process. It can be seen from these two figures that the light output power versus drive current characteristics of the VCSELs manufactured using the MBE process are fairly uniform. The light output power versus drive current characteristics of the VCSELs manufactured using the MOCVD process are quite non-uniform. Indeed, the light output power for the VCSELs manufactured using the MOCVD techniques exhibits a maximum output power differential of two milliwatts of output power at 21 milliamps of drive current. This wide variation of output characteristics between VCSELs in the same array can prove unacceptable for typical data communications applications.
Nevertheless, it is highly desirable to manufacture VCSEL arrays using the vapor phase epitaxial processes. The cost of manufacturing VCSEL arrays using MOCVD and MOVPE machines is considerably less than the cost associated with manufacturing VCSEL arrays using an MBE machine. MOCVD and MOVPE systems typically have higher throughput, shorter growth time, and a maximum down time of only about one day. Changing sources is generally much easier because there is no vacuum which must be vented during replacement. MBE machines, on the other hand, involve a high-vacuum system through which the atoms are introduced as beams transmitted to the wafers. If a failure occurs within the high-vacuum chamber, the entire system must be brought down, vented to atmosphere, the sources replaced, and a bakeout procedure must be performed at a temperature of about 200.degree. C. which takes as long as five days. Thus the down-time for an MBE system is typically on the order of one week. Moreover, the cost of MBE machines, particularly those which have higher wafer throughput and which employ load blocks for source replacement are far more costly than MOCVD and MOVPE type machines. For these reasons, MOCVD and MOVPE systems make up the bulk of the installed base of epitaxial machines in the industry.
Thus, it is highly desirable from a cost standpoint to find a solution to the high degree of array non-uniformity associated with the manufacture of semiconductor devices, particularly VCSELs, using MOCVD and MOVPE growth techniques.
It is known in the art that if light emitting devices are built using epitaxial growth techniques such as MBE, MOCVD, MOVPE, etc. on a substrate surface which is tilted to some degree with respect to the (100) crystallographic plane, that certain benefits can be derived. For example, U.S. Pat. No. 4,987,472 to Endo et al. discloses a method of manufacturing red and green LEDs by growing layers which form the LEDs on a substrate surface with an angle of inclination in the range of eight degrees to twelve degrees. The benefit derived from tilting the substrate surface is the elimination of surface defects which leads to higher light intensity from each of the LEDs produced on such a wafer.
In U.S. Pat. No. 5,016,252 to Hamada et al., a VCSEL device is disclosed for producing visible red light which is grown on a substrate surface having an angle of between five and seven degrees in the direction of the (011) plane. Again the purpose of the misorientation of the substrate is to reduce defect densities thereby decrease the threshold current for the VCSEL.
In U.S. Pat. No. 5,314,838 to Cho et al., a VCSEL for emitting infrared light is disclosed having a substrate misoriented by between one and seven degrees in the {111}A direction. The purpose of this misorientation is to grow distributed Bragg reflectors with improved reflectivity due to the elimination of roughness between the layers.
In the publication by G. B. Stringfellow et al., J. Electronic Materials, Volume 1, page 437 (1972), he discloses a phenomenon first experienced in building red edge emitting lasers because it is unique to the materials employed to emit red light. Red lasers typically employ GaInP/AlGaInP material systems. Stringfellow, et al. discloses that the Ga and In atoms tended to order themselves during epitaxial crystal growth such that In atoms will order themselves entirely in one plane while the Ga atoms order themselves entirely in another plane. A simple representation of this phenomenon is shown in FIG. 3. Instead of a distribution of In atoms mixed with Ga atoms to produce the desired alloy ratio, the In atoms are entirely separate in one plane 30 while the Ga atoms reside entirely in another plane 32, a layer 34 of atoms is sandwiched there between.
This long-range ordering problem tended to increase threshold current for the edge emitter and also its power consumption. Once the long-range ordering was identified using standard characterization techniques such as transmission electron microscopy (TEM), a solution to the problem was proposed to misorient the substrate. The misorientation of the substrate tends to create atomic steps of the lattice shown in FIG. 4. The more misoriented the substrate, the greater the number of atomic steps on the surface. Thus, when the edge emitter was grown on the misoriented substrate, although the Ga and In atoms still tended to order themselves along the steps, the existence of the steps increased the potential number of locations for the Ga and In atoms to be incorporated. This resulted in a more random distribution of the alloy atoms which led to lower power consumption and lower threshold currents. The long-range ordering effects in the InGaP material system were easily seen using known techniques for evaluating crystal growth. Moreover, the problem was reflected in the operating characteristics of the edge emitting laser despite its tendency to average out the effects of shorter-range variations in crystal composition because of the extensiveness of the ordering along the length of the edge emitting device.
When people began manufacturing VCSELs using the same red light material systems, they naturally continued to use misoriented substrates in the range of fifteen degrees (common misorientation of substrates needed to eliminate ordering in red edge--emitting lasers) because they were already familiar with the fact that this ordering phenomenon occurred in edge emitting devices and thus there was no reason not to continue to use the misoriented substrate for producing red VCSELs. With respect to VCSELs using GaAs and AlGaAs materials for emitting infrared radiation, no such long term ordering occurs and thus there has been no motivation to greatly misorient the substrate of infrared VCSELs. Moreover, it is only recently that VCSELs have been manufactured in the form of arrays, for which device-to-device uniformity has become a critical issue.